A topic that I sometimes bring up when people discuss how certain characters in the latest novel adaptation to anime isn’t fleshed out sufficiently is that they’re not supposed to be fleshed out, because they’re a supporting character, only there to help the main character’s fleshing out as they interact with them. In most novels, it’s very clear who the protagonist is, and it’s often clear that other characters not only aren’t protagonists, but they might not even be main characters at all.
In anime, these novels, and often manga (where ensemble casts are slightly more common), get posed as stories with a handful of main characters, often between three and five (three is a particularly common number for romantic series), but let’s take a look at Sound! Euphonium (Hibike! Euphonium in Japanese), where if we go by popular site MyAnimeList (MAL), then we have four, and those are also the first four appearing on the Wikipedia page for the show. All is content added by private individuals, but considering these four appear on the anime’s poster (and are four of the five characters appearing on the first novel’s cover as well), we could go by that.
(This is a Things I Like post, it’s not a review, but more a discussion of the show and of ideas that rose in my mind as a result of watching the show. There will be spoilers for the entire show.)